79 



insist on an absolutely clean condition of all nursery stock or 

 other plants destined for importation into their respective coun- 

 tries. It was certainly quite impossible for them to determine 

 experimentally the harmlessness of any species for so extensive, 

 variable, and complex a country as the United States. 



The discussion on Mr. Rogers's paper was then closed, and 

 F. V. Theobald gave his paper entitled : 



Aphides of the Cultivated Peas, and the Allied 

 Species of the Genus Macrosiphum. 



Revision of the British species of Macrosiphum ; distribu- 

 tion, habits, enemies, and treatment (cf. Vol. II., p. 380). 



Discussion. 



L. O. Howard heartily congratulated Mr. Theobald on 

 his important paper. The systematic study, with its result in 

 the way of reducing the number of host plants, was a beautiful 

 example of the value of such study. 



In America the pea Aphis was largely controlled by parasites. 

 He expressed surprise at the absence of parasites and interest 

 in the work of birds. Could it be possible, he asked, that the birds 

 had exterminated the parasites in this country ? 



Gordon Hewitt thanked Mr. Theobald for his important 

 paper, which indicated how dependent was the systematist on 

 the knowledge of the bionomics of an insect. He referred to the 

 occurrence of Macrosiphuui pisi in Canada, where it was not 

 infrequently controlled by its parasites. He also discussed the 

 methods of control. 



W. E. COLLINGE spoke of the value of certain birds which 

 feed on Macrosiphum pisi, particularly the Yellow Bunting, Blue 

 Tit, and Whitethroat. 



A. T. Gillanders said that Mr. Theobald's paper clearly 

 showed the value of systematic study to the economic student. 

 The nature of the life-history of the insect under consideration 

 showed that the creature migrated from plant to plant, and the 

 work of the economic student would be of little value if he were 

 not able from systematic study to determine the same or other 

 insects as associated with special food plants. 



